Abstract: Anterior cruciate ligament reconstruction surgery results in protracted quadriceps weakness that is associated with altered biomechanics and heightened risk of subsequent injuries. Little progress has been made in improving quadriceps strength in the past 15-20 years. Alterations in the morphological and cellular composition of the vastus lateralis muscle promote muscle weakness following an anterior cruciate ligament reconstruction. These features are not responsive to traditional physical therapy techniques. A promising emerging treatment, blood flow restriction training (BFRT) has been shown to address these same alterations in healthy subjects; however, it has never been tested in those with an injury. Pilot data in the current proposal shows that BFRT can lead to a significant increase in muscle strength following an anterior cruciate ligament injury. This project seeks to mechanistically determine the ability of BFRT to address underlying muscle cellular deficits following anterior cruciate ligament reconstruction compared to usual care. Goals in the current proposal are as follows: aim 1 will determine if BFRT is able to significantly improve knee mechanics and quadriceps strength following an anterior cruciate ligament reconstruction compared to usual care. Aim 2 will use advanced magnetic resonance imaging techniques to assess improvements in quadriceps morphology after training with BFRT. Lastly, aim 3 will identify BFRT-mediated improvements in cellular deficits in the quadriceps as the result of BFRT training versus usual care. The expected outcomes will show for the first time that BFRT is an effective mechanistic treatment for normalizing quadriceps strength within individuals following an anterior cruciate ligament reconstruction.